In order to operate modern internal combustion engines and to comply with strict emission limiting values, an engine controller may determine, by means of what is referred to as the cylinder charging model, the air mass which is enclosed in a cylinder per working cycle. In accordance with the modeled air mass and the desired ratio between the air quantity and fuel quantity (lambda), the corresponding fuel quantity setpoint value (MFF_SP) is injected via an injection valve which is also referred to in this document as an injector or direct injection valve. As a result, the fuel quantity which is to be injected can be dimensioned in such a way that a lambda value which is optimum for the post-treatment of an exhaust gas in the catalytic converter is present. For direct-injection spark ignition engines with internal mixture formation, the fuel may be injected directly into the combustion chamber with a pressure in the range from 40 to 200 bar.
Typically, the main requirement of the injection valve is, in addition to a seal against uncontrolled outflow of fuel and conditioning of the jet of the fuel to be injected, also chronologically precise metering of the pilot-controlled injection quantity. In particular in the case of supercharged direct-injection spark ignition engines, a very large quantity spread of the required fuel quantity may be necessary. It may therefore be necessary, for example for the supercharged operation at full load of the engine, to meter a maximum fuel quantity MFF_max per working cycle, while during operation near to idling a minimum fuel quantity MFF_min has to be metered. The two characteristic variables MFF_max and MFF_min here define the limits of the linear working range of the injection valve.
For direct injection valves with a solenoid drive, the spreading of a quantity, which is defined as the quotient between the maximum fuel quantity MFF_max and the minimum fuel quantity MFF_min at constant fuel pressure, is approximately 15. For future engines with the focus on CO2 reduction, the cubic capacity of the engines is reduced and the rated power of the engine is maintained or even raised by means of corresponding engine supercharging mechanisms. As a result, the requirement for the maximum fuel quantity MFF_max corresponds at least to the requirements of an induction engine with a relatively large cubic capacity. However, the minimum fuel quantity MFF_min is typically determined by means of operation near to idling and the minimum air mass in the overrun mode of the engine which is reduced in cubic capacity, and said minimum fuel quantity MFF_min is therefore reduced. In addition, direct injection permits the total fuel mass to be distributed over multiple pulses, which permits more stringent emission limiting values to be complied with, for example in a catalytic converter heating mode by means of what is referred to as mixture stratification and a later ignition time. For the abovementioned reasons, a raised requirement both in terms of the quantity spread and the minimum fuel quantity MFF_min may occur for future engines.
In known injection systems, a significant deviation of the injection quantity from the nominal injection quantity typically occurs in the case of injection in the range of a minimum fuel quantity. This systematically occurring deviation is due essentially to fabrication tolerances at the injector, as well as to tolerances of the output stage which actuates the injector in the engine controller. Further additional associated influencing variables are the fuel pressure, the cylinder internal pressure during the injection process and possible variants of the supply voltage.
The electrical actuation of a direct injection valve usually takes place by means of current-regulated full-bridge output stages of the engine controller. A full-bridge output stage makes it possible to apply a voltage of the on-board power system of the motor vehicle, and alternatively a boosting voltage, to the injection valve. The boosting voltage is frequently also referred to as a boost voltage (U_boost) and may be, for example, approximately 60 V.
FIG. 5 shows a typical current actuation profile I (thick continuous line) for a direct-injection valve with a solenoid drive. FIG. 5 also shows the corresponding voltage U (thin continuous line) which is present at the direct injection valve. The actuation is divided into the following phases:
A) Pre-Charge-Phase:
During this phase with the duration t_pch, the battery voltage U_bat, which corresponds to the voltage of the on-board power system of the motor vehicle, is applied to the solenoid drive of the injection valve by the bridge circuit of the output stage. When a current setpoint value I_pch is reached, the battery voltage U_bat is switched off by a two-point regulator, and after a further current threshold has been undershot U_bat is switched on again.
B) Boost-Phase:
The pre-charge phase is followed by the boost phase. For this purpose, the boosting voltage U_boost is applied to the solenoid drive by the output stage until a maximum current I_peak is reached. The rapid current build-up speeds up the opening of the injection valve. After I_peak has been reached, there follows a free-wheeling phase up to the expiry of t_1, during which free-wheeling phase the battery voltage U_bat is again applied to the solenoid drive. The time period Ti of the electrical actuation is measured from the start of the boost phase. This means that the transition to the free-wheeling phase is triggered by the predefined maximum current I_peak being reached. The duration t_1 of the boost phase is permanently predefined as a function of the fuel pressure.
C) Commutation Phase:
After t_1 expires there follows a commutation phase. Switching off the voltage results here in a self-induction voltage which is limited essentially to the boosting voltage U_boost. The limitation of the voltage during the self-induction is composed of the sum of U_boost as well as the forward voltages of a recuperation diode and of what is referred to as a free-wheeling diode. The sum of these voltages is referred to below as the recuperation voltage. On account of a differential voltage measurement, on which FIG. 5 is based, the recuperation voltage is in a negative form in the commutation phase.
The recuperation voltage results in a flow of current through the coil, which flow reduces the magnetic field to a minimum. The commutation phase is timed and depends on the battery voltage U-bat and on the duration t_1 of the boost phase. The commutation phase ends after the expiry of a further time period t_2.
D) Holding Phase:
The commutation phase is followed by what is referred to as the holding phase. Here, again the setpoint value for the holding current setpoint value I_hold is adjusted using the battery voltage U_bat by means of a two-point regulator.
E) Switch-off Phase: Switching off the voltage results in a self-induction voltage which, as explained above, is limited to the recuperation voltage. This results in a flow of current through the coil, which flow then reduces the magnetic field. After the recuperation voltage, which is in a negative form here, has been exceeded, no current flows anymore. This state is also referred to as “open coil”. Owing to the ohmic resistances of the magnetic material, the eddy currents which are induced during the field reduction of the coil decay. The reduction in the eddy currents leads in turn to a change in the field of the solenoid and therefore to a voltage induction. This induction effect leads to the voltage value at the injector rising to zero starting from the level of the recuperation voltage in accordance with the profile of an exponential function. After the reduction of the magnetic force, the injector closes by means of the spring force and the hydraulic force caused by the fuel pressure. The injected fuel quantity is therefore overall a function of the valve-opening behavior and valve-closing behavior as well as of the fuel pressure present at the valve.